Sugar, Uric Acid, and the Etiology of Diabetes and Obesity Richard J

PERSPECTIVES IN DIABETES Sugar, Uric Acid, and the Etiology of Diabetes and Obesity Richard J. Johnson,1,2 Takahiko Nakagawa,1,3 L. Gabriela Sanchez-Lozada,4 Mohamed Shafiu,5 Shikha Sundaram,6 Myphuong Le,1 Takuji Ishimoto,1 Yuri Y. Sautin,7 and Miguel A. Lanaspa1 The intake of added sugars, such as from table sugar (sucrose) For example, a high intake of fructose induces leptin re- and high-fructose corn syrup has increased dramatically in the sistance in rats (7). Fructose also encourages food intake last hundred years and correlates closely with the rise in obesity, due to stimulation of dopamine in the mesolimbic system metabolic syndrome, and diabetes. Fructose is a major compo- and effects on the hypothalamus (8,9). Food intake is also nent of added sugars and is distinct from other sugars in its ability stimulated by hepatic ATP depletion (10), which occurs in to cause intracellular ATP depletion, nucleotide turnover, and the animals and humans administered fructose (11). Fructose generation of uric acid. In this article, we revisit the hypothesis may also affect metabolic rate. A recent study in humans that it is this unique aspect of fructose metabolism that accounts documented a reduction in resting energy expenditure for why fructose intake increases the risk for metabolic syndrome. Recent studies show that fructose-induced uric acid in overweight and obese subjects fed fructose but not generation causes mitochondrial oxidative stress that stimulates glucose (12). fat accumulation independent of excessive caloric intake. These studies challenge the long-standing dogma that “a calorie is just FRUCTOSE-INDUCED METABOLIC SYNDROME DOES NOT a calorie” and suggest that the metabolic effects of food may REQUIRE INCREASED ENERGY INTAKE matter as much as its energy content. The discovery that fructose- mediated generation of uric acid may have a causal role in diabetes The ability for fructose (and sucrose, which contains and obesity provides new insights into pathogenesis and therapies fructose) to stimulate food intake and to lower metabolism for this important disease. Diabetes 62:3307–3315, 2013 provides a mechanism for how a high fructose intake may encourage weight gain and visceral fat accumulation. However, fructose or sucrose also alters fat stores and metabolism independent of excessive energy intake. Al- though weight gain is largely controlled by overall energy FRUCTOSE-INDUCED WEIGHT GAIN AND METABOLIC intake, other features of metabolic syndrome can occur SYNDROME independent of weight gain. For example, rats fed fructose develop fatty liver, hypertriglyceridemia, and insulin re- Experimental studies from the 1950s showed the peculiar sistance when compared with rats fed isocaloric glucose ability of fructose to induce insulin resistance in laboratory or starch-enriched diets (4,5). Indeed, hypertriglyceridemia, rats. Today, fructose intake has been shown to induce all fatty liver, and type 2 diabetes can be induced in metabolic features of metabolic syndrome in rats, as well as oxi- syndrome–prone rats with caloric restriction provided the dative stress, endothelial dysfunction, fatty liver, micro- fi diet is high (40%) in sucrose (which contains fructose) (5). albuminuria and kidney disease (rev. in 1). Similar ndings A recent epidemiological analysis in humans also found an can be shown when animals are fed sucrose or high-fructose association of diabetes prevalence with sugar availability corn syrup (HFCS), both which contain fructose (2,3). In that was independent of total energy intake (13). contrast, administration of glucose or starch results in fewer features of metabolic syndrome when provided equivalent intake (4,5). A ROLE FOR URIC ACID IN FRUCTOSE-INDUCED FAT Fructose may increase the risk for obesity by altering ACCUMULATION satiety, resulting in increased food intake. The intake of The observation that fructose-fed rats develop fatty liver fructose is not effective in stimulating insulin and leptin and metabolic syndrome without requiring increased en- secretion in humans, and hence may not induce a satiety ergy intake suggests that the metabolism of fructose may response (6). Other mechanisms may also be operative. be different from that of other carbohydrates. Fructose is distinct from glucose only in its initial metabolism. The first enzyme to metabolize fructose is fructokinase (also From the 1Division of Kidney Diseases and Hypertension, University of Colo- known as ketohexokinase [KHK]). The metabolism of rado Denver, Aurora, Colorado; the 2Division of Nephrology, Eastern Colo- rado Health Care System, Department of Veteran Affairs, Denver, Colorado; fructose to fructose-1-phosphate by KHK occurs primarily the 3TMK Project, Medical Innovation Center, Kyoto University, Kyoto, Ja- in the liver, is rapid and without any negative feedback, pan; the 4Laboratory of Renal Physiopathology and Department of Nephrol- and results in a fall in intracellular phosphate and ATP ogy, Instituto Nacional de Cardiologia I.Ch., Mexico City, Mexico; 5Renal 6 – Associates, San Antonio and Del Rio, Texas; the Division of Pediatric Gas- levels (14 16). This has been shown to occur in the liver troenterology, Children’s Hospital, Aurora, Colorado; and the 7Division of in humans with relatively small doses of oral fructose (60 Nephrology, Hypertension and Renal Transplantation, University of Florida, gfructosealoneor39gfructosewith39gglucose)(11). Gainesville, Florida. Corresponding author: Richard J. Johnson, [email protected]. The decrease in intracellularphosphatestimulatesAMP Received 1 February 2013 and accepted 17 June 2013. deaminase (AMPD), which catalyzes the degradation of DOI: 10.2337/db12-1814 AMP to inosine monophosphate and eventually uric acid Ó 2013 by the American Diabetes Association. Readers may use this article as (15) (Fig. 1). The increase in intracellular uric acid is long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by followed by an acute rise in uric acid in the circulation -nc-nd/3.0/ for details. likely due to its release from the liver (14). Fructose also diabetes.diabetesjournals.org DIABETES, VOL. 62, OCTOBER 2013 3307 FRUCTOSE, URIC ACID, AND DIABETES As such, this is an interesting condition in which marked nucleotide turnover and ATP depletion occur but without the ability to be further metabolized by this primary en- zymatic pathway to glucose, glycogen, or triglycerides (20). Nevertheless, the feeding of fructose to HepG2 cells lacking aldolase B resulted in a rapid accumulation of triglycerides, consistent with our findings that uric acid itself can induce triglyceride accumulation (19). These experiments explain why fatty liver and hyperuricemia are common complications of this disease (21) and also why fatty liver and diabetes are complications in subjects with glycogen storage disease I, in which hepatic intracellular ATP depletion and hyperuricemia also occur (22–25). Fi- nally, it provides an explanation for why fructose is lipo- genic (based on acetate labeling studies) despite little of the fructose molecule being incorporated into the triglyceride molecule itself (19). We next addressed how the degradation of nucleotides might lead to fat accumulation. Specifically, our group and others have shown that AMPD counters the effects of AMP-activated protein kinase (AMPK) (26,27). Whereas activation of AMPK in hepatocytes induces oxidation of fatty acids and ATP generation, AMPD has opposite effects. Overexpression of AMPD in HepG2 cells blocks fatty acid oxidation and increases fat accumulation, whereas silencing AMPD blocks fructose-induced fat accumulation. The mechanism is mediated in part by the generation of uric acid, which inhibits AMPK (27). In addition to inhibiting AMPK, uric acid may stimulate FIG. 1. Fructose-induced nucleotide turnover. Fructose is rapidly hepatic lipogenesis (28). The mechanism appears to be me- phosphorylated in the hepatocyte by KHK to fructose-1-phosphate (F-1-P), which uses ATP as a phosphate donor. Intracellular phosphate (PO4) diated by uric acid–dependent intracellular and mitochon- levels decrease, stimulating the activity of AMP deaminase 2 (AMPD2). drial oxidative stress (28). Although uric acid is a potent AMPD2 converts AMP to inosine monophosphate (IMP). IMP is me- antioxidant in the extracellularenvironment,whenuricacid tabolized to inosine by 59 nucleotidase (59NT), which is further de- fi graded to xanthine and hypoxanthine by xanthine oxidase (XO), enters cells via speci corganicaniontransporters, it induces ultimately generating uric acid. an oxidative burst that has been shown in vascular smooth muscle cells, endothelial cells, adipocytes, islet cells, renal tubular cells, and hepatocytes (29–31). Uric acid–induced stimulates uric acid synthesis from amino acid pre- oxidative stress appears to be mediated by the stimulation cursors, such as glycine (17). of NADPH oxidase, which translocates to mitochondria Recent studies suggest that this “side event” in fructose (28,29,32). Uric acid can also generate triuretcarbonyl and metabolism may be critical for how fructose induces aminocarbonyl radicals as well as alkylating species upon metabolic syndrome. First, there are actually two KHK reaction with peroxynitrite and can also directly inactivate isoforms, and they differ in their ability to activate this nitric oxide (NO) to 6-aminouracil (33,34). pathway. KHK-C phosphorylates fructose rapidly, con- The induction of

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